1. Field of the Invention
The present invention relates to an image forming apparatus such as a complex machine equipped with at least one of a copier, a printer, a facsimile and a plotter, more specifically relates to an image forming apparatus using a process of applying or attaching a protecting agent to the surface of an image bearing member; and a process cartridge detachably mounted to the main body of the image forming apparatus.
2. Description of the Related Art
Conventionally, an image forming apparatus employing an electrophotographic process has a charging unit for charging the surface of a photoconductor as an image bearing member. As one of charging methods used with a charging unit, there is a charging method based on close-contact discharging, in which a charging member is placed in contact or non-contact with the surface of a photoconductor, and the surface of the photoconductor is charged by close-contact discharging.
In recent years, with increasing demands for higher quality of images, and downsizing of apparatuses, charging devices are more and more required to contribute to higher quality of images and the downsizing thereof. In view of the requirements, a charging device employing close-contact discharging, in which a charging member is placed and used in contact with or close to an image bearing member, is effective because it needs not to be placed in a large-size charging device.
In a charging device employing the close-contact charging (discharging), it is hard to uniformly charge the surface of a photoconductor due to a nonuniform contact between the charging member and the photoconductor or due to an amount of fluctuation of a gap between the charging member and the photoconductor in the case where noncontact charging method is employed. To overcome the drawback, recently, an AC-superimposed discharging method has been often used in which a direct current (DC) component is superimposed on an alternating current (AC) component.
The close-contact charging method in which an AC component is superimposed on a DC component can be said as an extremely advantageous technique in terms of downsizing of apparatus, formation of higher quality of images and giving high-durability of photoconductor, because a charging member and a photoconductor can be placed in noncontact manner while keeping the uniformity of charging.
Such a charging method in which an AC component is superimposed on a DC component, however, activates a photoconductor surface to increase an adhesion force between the photoconductor surface and a toner, and thus from the viewpoint of the cleanability, it is disadvantageous in the configuration. Moreover, since toner particles are made to be small in diameter and to be more spherical to obtain a high quality image, the cleanability tends to further degrade.
Furthermore, recent studies reveal that since use of a charging method based on close-contact discharging tends to cause deterioration of a photoconductor surface since the photoconductor surface and peripheral portions are concentrically discharged. The deterioration of the photoconductor surface due to close-contact discharging occurs even in the absence of members which make contact with the photoconductor, unlike deterioration due to mechanical abrasion.
Under application of an AC voltage, such a problem with a degradation of cleanability and abrasion resistance of the photoconductor is conspicuous. Therefore, there is a great need to satisfy both the cleanability and the abrasion resistance.
As a means for solving the problem, there are the following configurations disclosed: a protecting agent applying unit for applying a protecting agent onto a photoconductor is provided to reduce mechanical abrasion of the photoconductor (see Japanese Patent Application Laid-Open (JP-A) Nos. 2002-156877 and 2002-244516); a protecting agent applying unit for protecting the surface of a photoconductor against chemical deterioration (see JP-A Nos. 2004-341480 and 2005-115311); and a unit for applying a solid protecting agent, such as zinc stearate, to the surface of a photoconductor.
When a protecting agent is used, the photoconductor is smeared with the protecting agent. To solve this problem, there is known a method for adjusting the applied amount of a protecting agent.
JP-A No. 2005-070276 discloses a configuration of an image forming apparatus in which there are provided a protecting agent-coating mechanism and a unit for forming the applied protecting agent into a thin layer of uniform thickness, at the downstream of a cleaning unit.
These units exhibit an excellent effect of satisfying both the cleanability and the abrasion resistance of a photoconductor, but a new problem arises that the protecting agent itself passes through a cleaning blade and is attached to the charging member.
If a protecting agent that has passed through the cleaning blade is attached to and accumulated on the charging member, the protecting agent shows up as an undesirable abnormal image such as black streaks.
In order to solve the problem with abnormal images such as black streaks, the present applicant proposes in JP-A No. 2008-122869, a protecting agent removing unit configured to remove a powdery lubricant so as to prolong the life span of a charging member.
However, in this method, the protecting agent removing unit becomes gradually smeared with time, and the effect of removing the protecting agent is reduced, and this method has not achieved in sufficiently prolonging the life span of a charging member.
In the form of a process cartridge, there is a problem that although photoconductor itself has a long life, it is replaced with a new one in an early stage of life due to the smeared charging member. Accordingly, there is still room for research on units for prolonging the life span of the whole members disposed around a photoconductor.